Triggering Second Workflow

I am looking for some feedback/best practices that you have seen or implemented and had success with.

I have a large workflow (currently sitting at 88 actions within 8 states in a state machine) and I am going to break it into 2 parts. The process has a perfect place to stop, but I am wondering how to approach it.

Part 2 begins when we receive external documentation and equipment has been installed, but the business would like to track how much time there is between when Part 1 ends and Part 2 kicks off. There is no define timeline on how long it should take or has to take, so they want to understand how long and go from there. Typically I would leave it in one workflow or state machine so we can see it easily, but separating them out presents a bit of a snag.

I was thinking of kicking off Part 2 as soon as Part 1 ends and then using a "Wait for item update". This would give me a time frame of how long it took to start Part 2. The issues I see with this is I will have the workflow running for (potentially) some unknown time before anything happens' taking up resources and possibly failing.

If I simply start Part 2 on when we receive the documents, they will need to do some manual calculations for each item. I could use some time stamps and update the list (Part 2 start - Part 1 end = time between parts).